The following invention relates to a point of use instantaneous water heating system for boosting the temperature of a conventional hot water supply to a temperature necessary for high volume, high temperature use such as commercial dishwashing.
Instantaneous water heating systems are needed for many applications including, for example, dishwashing in commercial food establishments. Since a conventional water heating system is designed for the purpose of supplying water at temperatures of only about 140.degree. F. to an entire facility, it has at times been necessary to install auxiliary water heaters for those applications which require water that is much hotter. Food service establishments, in order to meet local health regulations, must sometimes have dishwashing facilities capable of washing dishes in water that has been heated to approximately 180.degree. to 190.degree. F. This requires the use of such an auxiliary system, which is usually installed downstream from the conventional water heater in order to boost the temperature of the water from the conventional water heater to the desired level.
In order to provide sufficient water on demand, heated to the necessary temperature for these specialized processes, it has been necessary in the past to do one or a combination of the following:
(1) heat all water used in the facility to this higher temperature; or
(2) provide a complicated and unreliable valving system to separate the temperature ranges used for each process; or
(3) provide a separate, large-tank, gas-fired heating system for the higher temperature usage; or
(4) provide a separate, large-tank, electrically-powered heating system for the higher temperature usage (this option, while convenient, has historically resulted in very expensive operating costs and high maintenance costs).
Demand at the use point varies with time. In those high-temperature systems employing an accumulator tank, if the tank were simply to store the heated water when there is no demand at the use point, temperatures in the accumulator tank would decrease with time because of heat loss from the tank. Accordingly, the conventional practice is to replace the lost heat by intermittent reheating. This is conventionally achieved by recirculating the water from the accumulator tank through a high-energy instantaneous heater, preferably of the gas-fired type. If a small accumulator tank is used, water in the recirculation loop can become superheated, possibly causing an explosion in the instantaneous heater or accumulator. Thus, in the past, accumulator tanks for high-temperature systems have been made very large in order to accommodate a relatively large volume of water whose temperature remains more stable during recirculation than would the temperature of water stored in a smaller accumulator tank with less volume capacity.
The problem with the approach described above is that many establishments do not have the space for a large volume accumulator tank in the vicinity of the use point. Moreover, the operating costs of such large-tank systems are quite high.
Another problem with such prior systems is excessive fluctuation in the temperature of water supplied during periods of demand. The high-efficiency instantaneous auxiliary heater is conventionally operated according to a fluctuating duty cycle in response to output temperature to prevent overheating of the water. However, unless the heated water is passed through a large accumulator on its way to the use point, the non-correspondence of the usage load and the fluctuating duty cycle of the heater causes corresponding fluctuations in water temperatures and pressures at the use point. Since the 180.degree. to 190.degree. F. and 50 to 150 psi operating conditions of these systems is very close to the boiling point of water, often these conditions cause the release of high temperature and pressure water and/or steam through the required temperature and pressure relief safety equipment.
What is needed, therefore, is a point of use instantaneous water heating system that is compact and safe and provides hot water on demand at a predetermined, relatively constant temperature.